1. Field of the Invention
The present invention relates generally to optical devices such as lasers, and fiber optic data transmission systems employing the same, and particularly to a novel wavelength-locked loop servo-control circuit applied for writing control of Bragg gratings implemented in fiber optical links.
2. Description of the Prior Art
Wavelength Division Multiplexing (WDM) and Dense Wavelength Division Multiplexing (DWDM) are light-wave application technologies that enable multiple wavelengths (colors of light) to be paralleled into the same optical fiber with each wavelength potentially assigned its own data diagnostics. Currently, WDM and DWDM products combine many different data links over a single pair of optical fibers by re-modulating the data onto a set of lasers, which are tuned to a very specific wavelength (within 0.8 nm tolerance, following industry standards). On current products, up to 32 wavelengths of light can be combined over a single fiber link with more wavelengths contemplated for future applications. The wavelengths are combined by passing light through a series of thin film interference filters, which consist of multi-layer coatings on a glass substrate, pigtailed with optical fibers. The filters combine multiple wavelengths into a single fiber path, and also separate them again at the far end of the multiplexed link. Filters may also be used at intermediate points to add or drop wavelength channels from the optical network.
As known, one optical network system element includes a Bragg grating which is a short section of optical fiber that has been slightly modified.
Particularly, as illustrated in FIG. 1, in a portion 100 of optic fiber implementing a Bragg grating comprises cladding layers 110, 111 and, a core 112 forming an optical cavity having the fibre gratings 115. To form the gratings, the optical fiber core 112 at that portion 100 is exposed to ultraviolet radiation in a regular pattern, which results in the refractive index 119 of the fiber core to be altered according to that regular pattern. If the fiber is then heated or annealed for a few hours, the index changes become permanent. As described in K. Hill, Fiber Bragg Gratings, Chapter 9 in Handbook of Optics vol. IV, OSA Press (2000) and, B. Poumellec, P. Niay, M. Douay et al., xe2x80x9cThe UV induced Refractive Index Grating in Ge:SiO2 Preforms: Additional CW experiments and the macroscopic origins of the change in indexxe2x80x9d, Journal Of Physics D, App. Phys. Vol. 29, p. 1842-1856 (1996), the contents and disclosures of which are incorporated by reference herein, this phenomena is known as xe2x80x9cphotosensitivity.xe2x80x9d It is understood that the magnitude of the index change may depend upon many factors including: the irradiation wavelength, intensity, and total dose, the composition and doping of the fiber core, and any materials processing done either prior or subsequent to irradiation. For example, in germanium-doped singlemode fibers, index differences between 10xe2x88x923 and 10xe2x88x925 are achievable. Using this effect, periodic diffraction gratings can be written in the core of an optical fiber. Typically, the exposure is carried out using an interferometer or, through a phase mask with a periodic structure that permits writing of a periodically varying refractive index grating within the photorefractive media within the core. The reflectivity, bandwidth and central wavelength of such a Bragg structure are generally defined by the period and length of the phase mask and exposure time used.
Light traveling through these refractive index changes of optical fiber core having a fibre Bragg grating is reflected back, with a maximum reflection usually occurring at one particular wavelength known as the xe2x80x9cBragg wavelengthxe2x80x9d. That is, such gratings reflect light in a narrow bandwidth centered around the Bragg wavelength, xcexB, according to the following equation:
xe2x80x83xcex9B=2Neff
where xcex9 is the spatial period, or pitch, of the periodic index variations and Neff is the effective refractive index for light propagating in the fiber core. Thus, the wavelength of light reflected back depends on the amount of refractive index change that has been applied and also on how distantly spaced the refractive index changes 119 are. If the spacing of the Bragg planes is varied across the length of the grating, it is possible to produce a chirped grating, in which different wavelengths can be considered to be reflected from different points along the grating.
These in-fiber Bragg gratings written with photorefractive interference techniques have become an important part of modern fiber optic data communication systems. Such gratings have been employed in many systems, and are especially attractive for dense wavelength multiplexing (DWDM) where they can serve as in-line filters. In this capacity, the fiber Bragg grating functions as a wavelength-selective optical filter.
As the grating is produced by direct optical writing in a photorefractive fiber media, it is often difficult to control the alignment between the grating period (wavelength responsivity) and the center wavelengths of the DWDM communication channels. This mismatch can result in excessive optical loss and poor link performance.
It would be highly desirable to provide a system and methodology that ensures wavelength alignment between the filter bandpass with the center wavelength of the DWDM channel by monitoring the transmission properties of the grating as it is formed in the optical fiber.
It would be further highly desirable to provide a system and methodology for forming a Bragg grating in a length of optical fiber that employs a feedback loop for adjusting the writing laser as required to optimize the grating properties of the optical fiber in the grate writing process.
As known, fibre Bragg gratings may additionally be implemented as narrowband retroreflectors for providing feedback at a specific wavelength in fibre lasers (both in short pulse and single frequency lasers); filters for multichannel wavelength-division multiplexed (WDM) communications systems; and, fibre dispersion compensators in fibre links, or spectral manipulators of optical pulses as in a chirped pulse amplification (CPA) system. Fiber dispersion is a phenomena that causes optical pulses to spread as they propagate through fibers, eventually causing intersymbol interference and bit errors. It is important that effective compensation techniques be provided as dispersion is a fundamental limitation on the maximum data rate in a fiber optic communication systems. Fiber Bragg gratings applied for dispersion compensation may be re-written in real time using various schemes including: photorefractive or photocehemically induced schemes, or electrorefractive schemes. One company, Southampton Photonics, Inc., produces electrically Fiber Bragg Grating filter devices (http://www.southamptonphotonics.com). Digilens Inc. (http://www.digilens.com/) has developed electrically-switchable Bragg gratings (S-bugs(trademark)) in liquid crystals rather than solid substrates such as silica and silicon. As the characteristics of the liquid crystal can be modified by applying an electric current, Digilens Bragg grating devices may split off a specific wavelength and then adjust its power or switch it in a single operation. This may significantly increase the unrepeated link distance and improve the bit error rate for channels running at 1 to 10 Gbit/s or beyond. Further, tunable fiber Bragg gratings provide the means to change the grating period in response to external optical signals.
It would be further highly desirable to provide an improved tunable Bragg grating technology that incorporates a novel feedback control loop that would permit the automatic adjustment of the grating properties over time and as a function of optical power and wavelength, effectively allowing the control loop to correct for all wavelength dependent absorption or dispersion properties of an DWDM fiber link.
It is therefore an object of the present invention to provide a system and method for manufacturing a Fibre Bragg grating within an optical fiber employed in a WDM/DWDM communication system that enables reduction of the loss caused by mismatch of the alignment between the grating period (wavelength responsivity) and the center wavelengths of the WDM and DWDM communication channels.
It is another object of the present invention to provide a system and method that ensures wavelength alignment between the filter bandpass with the center wavelength of the DWDM channel by monitoring the transmission properties of the grating as it is formed in the optical fiber.
It is a further object of the present invention to provide a system and method for forming a Bragg grating in a length of optical fiber that employs a servo/feedback loop, referred to as a xe2x80x9cwavelength-locked loop,xe2x80x9d that enables dynamic adjustment of the writing laser as required to optimize the grating properties of the optical fiber during the grate writing process.
It is yet a further object of the present invention to provide an active dispersion compensation system for an optical network that enables re-writing, in real time, of the fiber Bragg gratings applied for wavelength dispersion compensation in fiber optic links using various optical schemes. Advantageously, this active dispersion compensation system may significantly increase the unrepeated link distance and improve the bit error rate for channels running at 1 to 10 Gbit/s or beyond.
It is still a further object of the present invention to provide a system and method for providing an improved tunable Bragg grating technology that incorporates a novel servo/feedback loop, referred to as a xe2x80x9cwavelength-locked loop,xe2x80x9d that permits the automatic adjustment of the grating properties over time and as a function of optical lower and wavelength, thereby effectively allowing the servo/feedback loop to correct for all wavelength dependent absorption or dispersion properties of an WDM/DWDM fiber link, thereby enabling significantly larger link power budgets and longer supported distances.
It is yet still another object of the present invention to provide a servo/feedback loop, referred to as a xe2x80x9cwavelength-locked loop,xe2x80x9d that provides compensation for wavelength dispersion effects while writing a fiber Bragg grating and enables control of the properties of the writing laser (intensity, wavelength, modulation period, etc.) so that the resulting grating will have a desired optical transfer function.
Thus, according to one aspect of the invention, there is provided a system and method for writing a Bragg grating pattern to an optical fiber providing a communication channel for an optical network, the method comprising: providing an optical signal capable of being communicated via a fiber optic link element providing said communication channel, the optical signal characterized as having an operating center wavelength associated with the channel; writing a grating pattern on a portion of the optical fiber link, the grating pattern providing a peaked wavelength selective function including a center wavelength; and providing real-time adjustment of the grating pattern being written to the fiber optical link element by the source to thereby mutually align the center wavelength of the peaked wavelength selective function resulting from the grating with the center wavelength of the optical signal, wherein the resulting grating writing pattern enables optical signals to be optimally communicated over the communication channel.
According to another aspect of the invention, there is provided a system and method for adaptively compensating for dispersion effects in optical fiber elements, the optical fiber element having formed therein a tunable Bragg grating pattern providing a peaked wavelength selective function including a center wavelength, the method comprising the steps of: providing an optical signal capable of being communicated via a fiber optic link element providing a communication channel, the optical signal characterized as having an operating center wavelength associated with the channel; providing a grating writing source for re-writing the Bragg grating pattern on a portion of the optical fiber link; and, enabling real-time adjustment of the grating pattern being re-written to the fiber optical link element by the source to thereby mutually align the center wavelength of the peaked wavelength selective function resulting from the grating with the center wavelength of the optical signal, the resulting re-written grating pattern eliminating dispersion compensation effects of the communication channel.
When implemented for writing Bragg grating patterns in optical fiber links or as an aid for reducing dispersion effects in optical fiber links, a wavelength-locked loop servo-control circuit is implemented that comprises: a mechanism for applying a dither modulation signal at a dither modulation frequency to an optical signal to be communicated over the channel, and transmitting the dither modulated optical signal through the optical fiber link during writing (or re-writing) of the grating pattern; a mechanism for converting a portion of the dither modulated signal output from the optical fiber link portion into an electric feedback signal; a mechanism for continuously comparing the feedback signal with the dither modulation signal and generating an error signal representing a difference between a frequency characteristic of the feedback signal and a dither modulation frequency; wherein a grating (re-)writing source is responsive to the error signal for automatically adjusting a grating spacing formed in the optical fiber link according to the error signal, wherein the center wavelength of the optical signal and the center wavelength of the wavelength selective function resulting from the formed grating pattern become aligned when the frequency characteristic of the feedback signal is two times the dither modulation frequency.
Advantageously, the system and method of the present invention may be employed in many applications utilizing fiber Bragg gratings, including but not limited to: optical communications and optical sensors, such as tapped optical delay lines, filters, multiplexers, optical strain gauges, and others; and further, is especially advantageous for implementation in WDM and DWDM optical systems.